hehe-basicly you are pretty right here- the etp has in real life very little value , even when it should be possible to discuss it to death by theroretical means.
i also remember the manual calculation, even considering wind and calculating by nautical air and nautical ground miles and i hated it, further was never in a situation when this would help with a decision.
the pure etp does not take into account if the next airport is your best option in regards to emergency facilities, runway lenght, descend path and much more, it does not take into account if going there will result in an overweight landing, it does not even take into account the wind when calculated by fmc. its just a basic information.
depending on what why and where happened its alsways a manual decision what to do.
cheers !